If there are only two or three agencies in the entire area now covered by the United States, a conspiracy among them may be practical. If there are 10,000, then when any group of them start acting like a government, their customers will hire someone else to protect them against their protectors.
…My own guess is that the number will be nearer 10,000 than 3. If the performance of present-day police forces is any indication, a protection agency protecting as many as one million people is far above optimum size.